Filaments of tungsten wire are in widespread use for incandescent and fluorescent lamps. In recognition of the detrimental effects of impurities on the metallurgical properties of tungsten, lamp manufacturers take extraordinary steps to produce tungsten wire having a minimum amount of residual impurities. For example, tungsten ore of the highest purity is used, and, to prevent impurity contamination during processing, the reduction of tungsten oxide is generally carried out in tungsten boats. Furthermore, as a routine quality control procedure, impurity analyses are carried out on the material at various stages of the powder-metallurgy process.
While a high degree of care is exercised in the manufacture of the filament wire, the manufacture of the filaments themselves has remained basically unchanged for the past 40 years. The conventional manufacturing procedure, in general, does not take into account any impurities which are introduced into the filament nor the deleterious effects of these impurities.
Tungsten filaments for electric lamps are commonly manufactured in the form of coils or coiled coils, the latter comprising a coil which is itself coiled. The most commonly used method for making such filament coils comprises the following steps: (1) winding tungsten wire as a coil on an elongated mandrel; (2) annealing the coil while still on the mandrel by passing it through a hydrogen furnace maintained at an elevated temperature, usually about 1200.degree. C.; (3) cutting the mandrel and coil to the desired length of the individual filaments; (4) dissolving away the individual mandrels in a suitable acid such as hydrochloric acid; and (5) re-annealing the tungsten wire in wet hydrogen at an elevated temperature, usually around 1300.degree. C, for cleaning.
The two materials for mandrels in common use throughout the lamp industry are steel and molybdenum. Aside from economics, the choice of mandrel material is severely limited by a large number of technical requirements. The most significant of these requirements for the mandrel are: (1) high tensile strength is required for filament winding and annealing under tension without plastic deformation of the mandrel; (2) the melting point of the mandrel must be above the annealing temperature required to set the filament coil prior to cutting; (3) the temperature coefficient of expansion of the mandrel should be close to that of the filament coil at the annealing temperature; (4) an adequate amount of bonding is needed between the filament coil and the mandrel during annealing to assure the retention of coil geometry upon subsequent cutting of the mandrel into individual filaments; and (5) the mandrel must be capable of being dissolved chemically without affecting the tungsten coil. For the foregoing reasons steel mandrels have been and are currently being used almost universally for most coiled filaments while molybdenum mandrels are used for coiled-coil filaments which require annealing temperatures above the melting point of steel.
The use of steel for the mandrel material, although economical, is undesirable from the standpoint of quality of the filaments produced, since steel has an adverse effect on the metallurgical properties of the tungsten filament during the manufacturing process. The main reason is that in forming the bond between the coil and the mandrel during the annealing step, a small amount of iron inevitably diffuses into and embrittles the tungsten. To understand this it should be considered that in the heavily drawn tungsten wire used as lamp filaments, a fibrous substructure exists prior to recrystallization, the average subgrain size of this structure being less than one micron. It is well recognized that substitutional diffusion of elements, such as iron, in tungsten occurs much more rapidly along sub-boundaries and grain boundaries of the tungsten than within the grains through the normal lattice sites. Since the activation energy for volume diffusion (within the grains) -- being around 120 Kcal/mole for iron in tungsten -- is several times higher than that for interfacial diffusion (along boundaries), an appreciable amount of interfacial diffusion can occur readily at temperatures less than one-half that of the tungsten. Therefore for tungsten, which has a high concentration of sub-boundaries and grain boundaries per unit volume when formed as heavily drawn wire used in filaments, a substantial amount of iron diffuses into the tungsten preferentially along the boundaries during annealing at a relatively low temperature. Furthermore, a concentration gradient of iron in the tungsten coil also exists, which decreases from the inner coil surface, in contact with the mandrel, to the outer coil surface, which is never in contact with the mandrel.
For tungsten filaments made by conventional methods on a steel mandrel, it has been substantiated, by impurity analyses on annealed coils made after dissolving away the steel mandrel, that an appreciable amount of iron diffuses into the filament coil. The amount of iron present after annealing varies from one coil segment to another, and depends upon the prior history of the tungsten wire. For tungsten wire approximately 2.5 mils in diameter, annealing in the manner described above typically results in an increase in iron concentration up to 50-100 ppm (by weight), as compared with 10 ppm or less of iron in the wire prior to annealing. Analyses of the surface material etched off from the annealed coil shows concentrations of iron substantially above 100 ppm.
The presence of localized segregations of iron diffused into a tungsten wire filament coil has been found to be responsible for the formation of numerous slivers of iron on annealed filament coils after the mandrel has been dissolved away. When the coil is stretched out, the slivers are evident on the inner surface of the coil at the areas where it was originally in contact with the steel mandrel. The amount of slivering increases with increasing concentration of iron, and is absent wherever the diffusion of iron into the tungsten wire coil is prevented.
Tungsten wire filament coils having excessive segregations of iron are also brittle and contribute to shrinkage (rejects) in the manufacture of electric lamps. Fracture of the filaments frequently occurs at the inner side of the coil being clamped by the leads during mounting of the coil in a lamp. This is indicative of the strong embrittlement effect of localized iron, since the compressive stresses at the inner side of the coil should favor plastic flow instead of crack initiation.
Another detrimental effect of iron is to reduce the advantages achieved by doping in non-sag filaments. Incandescent lamp filaments are normally doped with small quantities of aluminum, silicon, and potassium compounds to raise the recrystallization temperature and to develop an interlocking grain structure characteristic of sag resistant tungsten at elevated temperatures. It is well known that iron diffused into doped tungsten reduces the recrystallization temperature and develops a non-interlocking equi-axed grain structure, partially nullifying the effect of dopants in producing a nonsag material.
In accordance with the present invention a new type mandrel is utilized for forming filament coils as well as a new method for manufacturing filament coils. These provide a filament in which the amount of impurities introduced therein during manufacture can be controlled and reduced. Filaments made in accordance with the subject invention have been found to have greatly improved operating characteristics, due to the reduction in the impurity content.
In accordance with a preferred embodiment of the invention, a material which has a lower melting point than the filament material and does not alloy therewith is coated over an inner core of the mandrel. The coil is then wound over the coated mandrel and is annealed. The annealed coil is cut into desired lengths and the mandrel is dissolved from the core. Upon annealing of the filament, the coating material forms a strong bond with the coil and serves as a barrier to the diffusion of the inner core material into the filament coil. This produces a filament having substantially no additional impurities diffused therein from what was present in the filament material prior to annealing. The coating material on the mandrel also eliminates the formation of slivers on and the embrittlement of the annealed coil. Further, the coating material also aids in speeding the dissolving process.
In a preferred embodiment of the invention, copper or a copper alloy is used as the coating material over an inner mandrel core of steel. The copper or copper alloy melts during the annealing of the filament coil and forms a bond therewith to provide a better geometrical set. The mandrels also can be dissolved very rapidly from the coils using a suitable acid, such as nitric acid.
An another aspect of the invention, the coating material can be applied to the wire rather than to the mandrel.
It is therefore an object of the present invention to provide an improved filament for use with electric lamps.
A further object is to provide an improved tungsten filament for an electric lamp which, after annealing, has substantially the same amount of impurities present as before annealing.